1. Field of the Invention
This invention relates generally to a method of forming articles by injection molding, and more particularly concerns a method and an apparatus for forming an optical disc such as video disc, DRAW disc or erasable disc by injection molding.
2. Description of the Prior Art
As an example of prior art for forming a video disc by injection molding, we may take U.S. Pat. No. 4,185,955. According to this prior art, melted plastic material is injected into a cavity formed between dyes which are in a mutually thrusted state (a state that a movable die is pressed against a stationary die at a high pressure of several hundred tons) in order to obtain a video disc into which signal patterns (pits) in a stamper are copied.
In forming optical discs such as the video disc by injection molding, it is important that signal patterns in a stamper are copied well (that the signal patterns in the whole area of the stamper from its center to its skirt are uniformly copied), that birefringence of the disc (which means, in this case, a phase shift between incident light beams and reflected light beams for reading signal patterns in a disc, and which are mainly caused by internal strains in the disc) is made small as much as possible when the disc is formed by injection molding, etc. When brefringence is large, the signal patterns in the disc cannot be read by a detector. Particularly, in-optical discs such as the DRAW disc for both signal writing and signal readout, it makes the matter very serious that written signals cannot be read later. In order to obtain a good copy from the stamper and make birefringence of the disc small, it becomes important how to form the disc.
Not a compact video disc (with a diameter of 12 cm) but a large video disc (with a diameter of 20 to 30 cm) is conventionally formed by injection molding using acrylic resin having a good moldability (which means that the melted plastic material has a good flowability in a cavity formed between dies, so that it becomes easy to mold), but there are a few problems as follows:
(A) Since acrylic resin is very hygroscopic, an aluminum film cannot be strongly adhered, so that it, for example, tends to peel off as time passes.
(B) Since acrylic resin is of low heat resistance, two discs cannot be heated to a high temperature when a double-sided disc is made through adhesion between the two discs so that adhesion strength between the two discs becomes low.
(C) Since acrylic resin is of low impact resistance, it can be easily damaged by the impact worked when dropped or other loads.
Thus, the small-sized compact disc is conventionally formed by injection molding using polycarbonate resin, as polycarbonate resin has a lower hygroscopicity, a higher heat resistance and a higher impact resistance as compared with acrylic resin. Therefore, if the large video disc is formed using polycarbonate resin, the above-mentioned problems (A), (B) and (C) can be overcome.
The present inventor conducted an experiment with forming a large-sized video disc using polycarbonate resin after the same method of injection molding as disclosed in U.S. Pat. No. 4,185,955.
However, the disc obtained as the result of this experiment had not only a poor copy of signal patterns of the stamper in its skirt portion, but also such large birefringence that only the portion on or near the radius of 125 cm of the disc satisfied a practical birefringence range of .+-.35.degree., so that signals could not be read therefrom.
It was considered such that poly-carbonate resin has low moldability as compared with acrylic resin and high photoelastic modulus, so that flowability of the melted plastic material in the cavity is largely varied in the radius direction of the disc. That is, in case of forming small-sized discs such as the compact disc, the melted plastic material has comparatively good flowability in the cavity and there is scarcely a pressure difference between the central portion and the skirt portion of the disc. On the contrary , in case of large-sized discs such as the video disc, the melted plastic material has considerably lower flowability in the cavity and undesirable stresses are generated in the melted plastic material in the cavity. As a result, polymer content and residual stressses are not uniformly distributed over the whole extent from the central portion to the skirt portion of the disc in the cavity, so that internal strains seem to be much generated.